1. Field of the Invention
The present invention relates to an opto-magnetic signal reproducing apparatus for reading, by a magneto-optical effect, information recorded on an opto-magnetic recording medium.
2. Related Background Art
An opto-magnetic signal reproducing apparatus magnetically records information by utilizing local temperature rise of a magnetic film by spot irradiation of a laser beam and reproduces the information by magneto-optical effect. The apparatus has a large capacity memory which is erasable and rewritable.
FIG. 1 shows an example of a conventional opto-magnetic signal reproducing apparatus and illustrates the configuration and principles of signal reproduction. In FIG. 1, numeral 21 denotes a semiconductor laser, numeral 22 denotes a collimator lens, numeral 23 denotes a polarization beam splitter, numeral 24 denotes an objective lens, numeral 5 denotes an opto-magnetic recording medium, numeral 26 denotes a focusing lens, numeral 27 denotes an analyzer, numeral 28 denotes a photo-detector and numeral 20 denotes a phase shifter. Assuming that a P polarization plane is in a direction P and a direction of polarization of the semiconductor laser 21 is in the P polarization direction, a light beam emitted from the semiconductor laser 21 is collimated by the collimator lens 22 and directed to the polarization beam splitter 23. A high C/N signal is produced when t.sub.p.sup.2 .about.70% and R.sub.s.sup.2 .about.100% where t.sub.p is an amplitude transmission rate of a P polarization component for the polarization beam splitter 23 and r.sub.s is an amplitude reflection rate of an S polarization component. The collimated light beam passed through the polarization beam splitter 23 is focused by the objective lens 24 on the opto-magnetic recording medium 5 as a fine spot.
The opto-magnetic recording medium 5 is constructed as shown in sectional view FIG. 2, in which numerals 11 and 17 denote transparent substrates, numerals 12 and 14 denote dielectric multi-layer films, numeral 13 denotes a magnetic film, numeral 15 denotes a metal reflection film and numeral 16 denotes a protective film.
The light beam is directed in a direction shown by arrow L and has a polarization plane thereof rotated oppositely in accordance with a direction of magnetization (magnetic domain) of a spot irradiated area (magneto-optic Kerr effect), and a Kerr rotation angle is amplified by the dielectric multi-layer films 12 and 14 and the metal reflection film 15. A tracking guide groove is usually formed in the transparent substrate 11, although it is omitted in FIG. 2.
The light reflected by the opto-magnetic recording medium 5 is collimated by the objective lens 24 of FIG. 1 and reflected by the polarization beam splitter 23. Thus, an apparent Kerr rotation angle is increased by a ratio of r.sub.p.sup.2 ; (wherein r.sub.p.sup.2 =1-t.sub.p.sup.2) and r.sub.s.sup.2. Then, the light is separated into a polarization component by the analyzer 27 through the focusing lens 26, and is directed to the photo-detector 28 as a signal light.
The reflected light directed to the analyzer has the polarization plane thereof rotated by .+-..theta.k shown in FIG. 3 in accordance with the magnetic domain on the medium. Accordingly, if a transmission axis direction is an angle .alpha. with respect to the S polarization direction, a light intensity detected by the photo-sensor 28 is the square of the normal projection component of the amplitude to the analyzer transmission axis, and a signal intensity-modified in accordance with the magnetic domain is read out. The signal light I is represented by EQU I.varies.R.sup.2 =(sin.sup.2 .alpha.+.theta.k sin 2.alpha.) . . .(1)
where R is a Fresnel reflection factor of the opto-magnetic medium. A second term of the formula (1) represents the opto-magnetic signal.
There is usually a phase difference between the P polarization beam and the S polarization beam reflected by the opto-magnetic recording medium. Thus, strictly speaking, a linear polarization beam shown in FIG. 3 is not produced, but an elliptic polarization beam is produced. When an ellipse rate is high, a C/N ratio (an S/N ratio in a carrier wave band) of a reproduced signal may be lowered. To prevent it, Japanese patent application Laid Open No. 20342/1985 proposed to insert a phase shifter as shown in FIG. 1 to reduce the phase difference.
On the other hand, in another approach to improve the C/N ratio of the reproduced signal, the reflection light from the opto-magnetic recording medium is split by a half-mirror and the split light beams are differentially detected through analyzers having different transmission axis directions. Thus, the signal light I' is represented by EQU I' .varies. R.sup.2 .theta..sub.K sin 2.alpha. . . . (2)
and the first term of the formula (1) is eliminated and a high C/N ratio signal is produced. Such a differential detection is disclosed in U.S. Pat. No. 4,558,440 issued on Dec. 10, 1985, FIGS. 8A and 8B and specification column 9, line 29 through column 10, line 25, U.S. Pat. No. 4,561,032 issued on Dec. 24, 1985, FIGS. 8A and 8B and specification column 9, line 26 through column 10, line 23; U.S. Pat. No. 4,569,035 issued on Feb. 4, 1986, FIGS. 7A and 7B and specification column 5, lines 9 through 24; and U.S. Pat. No. 4,599,714 issued on Jul. 8, 1986, FIG. 1 and specification column 1, lines 23 through 51.
However, the half-mirror for splitting the light beam usually has a property to cause a phase difference between the P polarization component and the S polarization component of each of the transmitted light and the reflected light. Accordingly, in the differential detection, even if the phase difference between the P and S polarization components by the recording medium is compensated by the phase shifter described in the above Japanese patent application, a phase difference is caused by the half mirrors between the P and S polarization components of each of the two split light beams and a difference results between opto-magnetic signal levels. As a result, the C/N ratio is lowered.